Reversible Reactions

Specification Point 4.13:
  • Recall that chemical reactions are reversible, the use of the symbol ⇌ in equations and that the direction of some reversible reactions can be altered by changing the reaction conditions.
  • Some reactions go all the way to completion and the reaction stops when all of the reactants are used up.
  • In reversible reactions, the product molecules can themselves either react with each other or decompose and re-form the reactant molecules again.
  • The reaction can occur in both directions: the forward reaction (which forms the products) and the reverse direction (which forms the reactants).
  • When writing chemical equations for reversible reactions, two arrows are used to indicate the forward and reverse reactions.
  • Each one is drawn with just half an arrowhead – the top one points to the right, and the bottom one points to the left:

A + 2B ⇌ AB2

  • The direction of reversible reactions can be altered by adjusting some or all of the following reaction conditions:
    • Temperature
    • Pressure
    • Concentration
  • The direction of change depends on the chemical nature of the reactants and products and their physical states and properties.

Dynamic Equilibrium

Specification Point 4.14:
  • Explain what is meant by dynamic equilibrium.
  • Reversible reactions can reach a state of dynamic equilibrium.
  • Equilibrium occurs when the forward and reverse reaction proceed at exactly the same rate.
  • Both reactions are taking place simultaneously but there is no overall effect on the concentrations of the reacting substances i.e. their concentrations remain constant at equilibrium.
  • Equilibrium is dynamic i.e: the molecules on the left and right of the equation are constantly chemically changing into each other.
  • It only occurs in a closed system so that none of the participating chemical species are able to leave the reaction vessel nor can any other substances enter.

Equilibrium Open & Closed, Edexcel GCSE Chemistry

Equilibrium can only be reached in a closed vessel which prevents reactants or products from escaping system

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Morgan Curtin Chemistry

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.